1. Field of the Invention
The present invention relates generally to the field of fiber optics, and in particular to connectors and connection techniques for multicore optical fibers.
2. Background Art
A multicore fiber (MCF) is an optical fiber in which a plurality of cores extend through a common cladding. MCF technology has the potential of greatly increasing the capacity of current optical fiber networks by allowing a plurality of optical data signals to be carried in parallel by a single fiber. One important technical issue to be addressed is connectorization, i.e., how to suitably terminate MCF cables with mating connectors that can be easily joined together with acceptably low insertion loss. It will be apparent that an important component in minimizing insertion loss is to connectorize MCF cables such that when a first mating connector and a second mating connector are joined together, the MCF cores at the first connector are properly aligned with respective MCF cores at the second connector, so as to preserve each of the parallel data transmission channels.
Techniques have been developed for terminating single-mode and multimode MCFs using standard simplex and multifiber connectors (i.e., connectors developed for use with single-core fibers). However, one drawback of prior art connectors of this type is their general inability to address the issue of insertion losses resulting from transverse displacements, or “offsets,” of one or more MCF cores at the first connector relative to the respective MCF cores at the second connector. Current limitations on the dimensional control of various features of MCFs, such as outside diameter, core position, and the like, result in optical insertion losses that are significantly higher than those typically achievable with comparable single-core fibers, given equivalent dimensional control of the alignment components, such as ferrules, alignment pins, and the like.
Transverse core offsets between opposing MCF cores in mated connectors can result, for example, if one or more of MCF cores are not in their ideal positions relative to the adjacent MCF cores, or if the clearance variations between the fiber outer diameter and the ferrule inner diameter are too large. In such cases, standard connector termination procedures cannot be used to minimize the offsets.
The issue of transverse core offsets also arises when using mated connectors in conjunction with “twisted” MCFs, i.e., an MCF in which the cores propagate helically around the fiber axis. When twisted MCFs are utilized, standard fiber termination and polishing procedures can cause excessive rotational misalignment of the cores. As material is polished away from the connector ferrule, the cores migrate away from their initial position. This is especially problematic in the case of angled physical contact (APC) connectors. Typically, a twisted MCF would be rotationally aligned and fixed relative to the housing prior to being angle polished. Angle polishing requires a relatively significant amount of material to be removed from the fiber and ferrule endface.
Twisted multicore fibers are typically used in shape-sensing applications (for medical procedures, bore-hole drilling, etc.), which employ optical backscatter reflectometers (OBR). The back-reflection performance of each connector can greatly affect shape-sensing system performance. Therefore, APC connectors, which provide a 65 dB minimum RL, versus the 50 dB RL provided by PC connectors, are typically specified. Thus, APC connector termination and polishing must be performed in a way that avoids excessive core misalignment, since the misalignment would result in large transverse offsets and high, inconsistent insertion-loss performance.